1. Filed of the Invention
This invention relates to a pneumatic tire, and more particularly to a pneumatic radial tire of so-called run-flat type capable of running over a given distance at a state that an inner pressure is zero or neat to zero due to puncture or the like. Particularly, the invention relates to a pneumatic radial tire having a relatively large ratio of section height to section width or an aspect ratio of not less than 60 and an excellent run-flat durability (running at puncture state).
2. Description of Related Art
The run-flat type radial tire (hereinafter referred to as run-flat tire) is mainly applied to vehicles such as passenger cars and the like wherein the bearing load of the tire is relatively small, and is required to satisfy a condition that even if the tire is suddenly rendered into a flat state (or puncture state) during running on general-purpose road or high-speed running on an expressway, it can stably be run without damaging the handling stability of the vehicle or the passenger car location. Particularly the tire can stably and surely be run up to a tire exchangeable over a given distance of, for example, 80-160 km without separating off from a used rim (or approved rim) and causing breakage even in the continuation of the running.
For this end, run-flat tires having various structures are proposed or sometimes in a combination of a fully worked-out rim to be used. The tires relating to these proposals are roughly divided into super-low section profile tires having an aspect ratio of less than 60 and tires having an aspect ratio of not less than 60 and being relatively high in the section height.
As to the super-low section profile tires, an example of a run-flat tire excellent in the cost performance and most practically used in markets is a tire having a structure that as a section of a tire 21 is shown in FIG. 5, a pair of thick reinforcing rubber strips 29 each having a crescent-shaped section are applied to an inner face side of an innermost carcass ply 26-1 extending from a bead portion 22 through a sidewall portion 23 to an end part of a tread portion 24. However, this type of the tire cannot avoid the rise of the cost, so that it is frequently mounted onto an expensive vehicle assuming high-speed running such as a sport car, a sport-type car and the like.
In the tire 21 having the thick reinforcing rubber strips 29, a radial carcass 26 has a two or more ply structure comprising a turnup ply 26-1 wound around a bead core 25 from inside of the tire toward outside and a down ply 26-2 enveloping the turnup ply 26-1 from the outside in order to mitigate a deformation degree crushed under flat running as far as possible. Also, a hard stiffener rubber 27 extending from an outer peripheral surface of the bead core 25 up to a position near a maximum tire width S is arranged so as to sandwich between the turnup ply 26-1 and the down ply 26-2. Further, a rubberized layer containing Kevlar cords or steel cords therein (called as an insert ply) is sometimes arranged so as to extend from the bead portion 22 to the sidewall portion 23.
Recently, tires having a relatively high section height as mentioned above are frequently mounted onto high-grade import passenger cars and high-grade domestic passenger cars having a relatively large displacement. In this case, it is possible to run the tire in cooperation with an installed rim at a run-flat state. A type rim type incorporating a tire support member into the rim is main.
In the tire having the structure shown in FIG. 5, in order to improve the durability in the run-flat running, there are proposed and practiced a method of increasing the thickness or height of the reinforcing rubber strip 29, a method of largely increasing the hardness or modulus of rubber itself and the like. In these methods, however, there are restrictions in the production, restriction on the rising breadth of the cost and the like. Therefore, when the inner pressure of the tire is suddenly reduced to zero, it is particularly necessity to ensure the handling stability of the vehicle during the high-speed running and also the durability in the continuation of the running at the run-flat state in this tire handling and durability is not sufficient, so that it is desired to develop a tire ensuring these performances and improving the run-flat durability.
In case of incorporating the tire support member into the rim, the assembling of the tire onto a wheel is not easy or there is caused a problem in so-called rim assembling property. Also, it is unavoidable to largely increase the weight in the tire-wheel assembly and the unsprung mass of the vehicle is highly increased to considerably degrade the ride comfort of the vehicle against vibrations. Therefore, there is raised a problem that the above incorporation of the tire support member is unsuitable for the high-grade passenger car.